The present invention relates to a switching arrangement in motor vehicles for the timed switching-on of inductive consuming devices via a semiconductor power switch timed by a control source.
A switching arrangement of the above-described type is known, for example, from the German Patent Document DE-38 35 662 A1, from the article "New Fast-Recovery Power Mosfets Improve the Efficiency in the Case of High Voltage Motor Controls" in the journal The Electronics Engineer, No. 5/1990, and from the article "MOSFETS Control Motors in Automatic Applications" in the Journal Electronic Components and Applications, Vol. 9, No. 2. Such arrangements are used for minimizing power loss or for high-frequency timing. As an example the teaching book Current Supply Switches, Triac- and Ignition Switches by D. Nuehrmann, 1984, page 189, generally makes known a switching arrangement for periodic switching-on of inductive consuming devices by means of semiconductor power switches, whereby the rise-time is lengthened by means of an increase of the internal resistance of the control source. This known switching arrangement, however, exhibits spurious oscillation caused by the components during the switching-through of the semiconductor power switch which negatively influence the leading edge of the switching-on signal (drain voltage) of the inductive consuming device.
When controlling inductive consuming devices in motor vehicles, such as electric motors, timing frequently takes place with high currents. In this situation, electromagnetic radiation will occur when screening measures are insufficient, in which case there may be considerable interference with electronic components. This radiation is caused by spurious oscillations when the semiconductor power switch is switched through. It also is caused by high-frequency components of the steep edges of the timing signals.
It is an object of the present invention to provide a switching arrangement of the initially described type by means of which the inductive consuming devices are timed with high currents and can be controlled free of electromagnetic radiation.
This and other objects are achieved by the present invention which provides a switching arrangement in motor vehicles for timed switching-on of inductive consuming devices. This switching arrangement includes a control source and a semiconductor power switch coupled to the control source and timed by the control source. A timing element of a cut-in circuit of the semiconductor power switch is smaller than or equal to a reaction time of the control source. The reaction time is significantly shorter than a rise time of a control voltage of the semiconductor power switch.
Each of the two time relationships predominantly counteracts one of the two above-mentioned causes of electromagnetic radiation respectively, as described in the following.
One of the problems--the component-caused spurious oscillations during the switching-through of the semiconductor power switch--is solved when the control source is switched off in such a manner that the same exhibits a reaction time with respect to the spurious oscillations, by means of which a compensation of these spurious oscillations is reached. In this case, the timing element of the cut-in circuit of the semiconductor power switch is the period of one of the spurious oscillations which occur at the switching-through moment of the semiconductor power switch from the charge reversal operations in the parasitic capacitances of the semiconductor power switch in interaction with the internal resistance of the control source. The smaller the internal resistance of the control source, the shorter the timing element of the cut-in circuit of the semiconductor switch. The reaction time of the control source is the shortest time within which the control source can compensate the spurious oscillations so that a linearly rising voltage is achieved at the output of the control source which is identical with the control voltage of the semiconductor power switch.
The second problem--the electromagnetic radiation by high-frequency parts of a timing signal with steep edges--is solved when the edges are flattened by means of an increase of the rise time until the radiation is minimized corresponding to the motor vehicle regulations. In this case, the rise time is the time within which the control source increases the control voltage of the semiconductor power switch uniformly from the minimal to the maximal level. The rise time is a multiple of the reaction time of the control source.
Accordingly, as a result of the time relationships corresponding to the invention, electromagnetic radiation is prevented from arising so that no cost-intensive measures must be taken in order to suppress it.
In an embodiment of the invention, the semiconductor power switch is a field effect transistor and the control source includes a power operational amplifier.
The use of a field effect transistor as the semiconductor power switch shortens the timing element of the cut-in circuit because of the fact that field effect transistors have no retention time and can be switched faster than, for example, bipolar transistors. By the use of a power operational amplifier in the control source, a very short reaction time is achieved in a simple manner. At the same time, the minimal low impedance of the internal resistance of a power operational amplifier provides the adjustment of the ratio according to the invention of the timing element of the cut-in circuit to the reaction time.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.